It’s a futon filled with feathers of water birds. Feathers having rachises are used for the fillings of hane-butons. In general, a futon filled with at least 50% feathers or more is called a hane-buton. The rachises inside the fillings make the elasticity of the futon and also make the rustling noise. Hane-butons are usually available at a lower price than umo-butons.